ON THE ANATOMY OF FISHES. 
71 
compressed, so as to present the appearance of a median bony keel when viewed 
from the ventral surface. The sixth centrum is much smaller, but nevertheless 
somewhat larger than the normal or but little modified centra that succeed it 
(figs. 1 to 3, v.^). The body of the seventh vertebra (v.'^) resembles the normal 
free vertebral centra, except that its ventro -lateral margins are over-lapped by the 
splint-like posterior prolongations of the superficial ossifications. The remaining 
vertebral centra are normal and free. 
The neural arches of the third and fourth vertebrae form the continuous bony walls 
of the neural canal in the region of the complex centrum ; posteriorly they appear to 
be completely anchylosed with the arch of the fifth vertebra, which in turn is 
partially confluent with the arch of the sixth vertebra, at all events only very faint 
indications of sutures can be detected between them (fig. 2). In young specimens 
the arches of the fifth and sixth vertebrae are readily separable from each other, and 
also as a rule from the arch of the complex vertebra, the completeness of the 
coalescence being largely dependent on the age of the particular specimen examined. 
The arch of the seventh vertebra is usually quite distinct from its fellows with which, 
like the normal free vertebrae, it articulates by means of pre- and post-zygapophyses, 
but in old specimens it also may become partially anchylosed to the arch in front. 
In a vertical longitudinal section of this part of the vertebral column, the bony 
walls of the neural canal are seen to be perforated on each side by successive pairs of 
small foramina for the exit of the dorsal and ventral roots of the fourth to the eighth 
pairs of spinal nerves inclusive (figs, 2, 5, see also p. 98). 
The second vertebra has no distinct neural spine. The spine of the third vertebra 
forms a thin vertical and much elongated lamina of bone, which, like its neural 
arch, is inclined forwards in front, and articulates with the exoccipitals and supra- 
occipital, and is also overlapped along the greater part of its free dorsal edge 
by a spine-like backwardly-projecting process, derived from the supraoccipital 
(figs. 1, 2, 4, and 5, n.s.^). I’he anterior margin of the lamina is much thickened, 
and, moreover, is slightly bifid, so as to clip, as it were, the posterior edge of the 
supraoccipital, at the junction of the latter with the two exoccipitals, but usually a 
small remnant of intercalated cartilage (fig. 5, it.c.) still persists between the two. 
The spine of the fourth vertebra is represented by a thickened spur of bone, of 
greater length than the foregoing, with which it is continuous anteriorly, and 
inclined obliquely backwards to such an extent as to overlap the much shorter and 
nearly vertical spines of the fifth and sixth vertebrae (fig. 2, n.s^). Its distal 
extremity is free and deeply cleft, so as to clip, and thereby su[>port, the central 
portions of the two anterior interspinous bones of the dorsal fin. The spinous 
processes of the fifth, sixth, and seventh vertebrae are much shorter, and so crowded 
together as to be with difiSculty distinguishable as separate elements, although 
sutures exist between them as well as between the spines of the fifth and fourth 
vertebrae. Their distal extremities are bifid, and support the proximal extremities of 
